This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Roux-en-Y gastric bypass is the most common type of obesity surgery. It induces both severe restriction of stomach capacity, and malabsorption. It is associated with greater weight loss, but also greater risk for complications than restrictive procedures (e.g. gastric banding). Possible adverse outcomes include protein deficiency, abnormal shifts in fluid distribution, and excessive loss of lean tissue. It is unclear why this surgery yields successful weight loss with minimal negative effects in some individuals, but not others. There is a need to identify factors associated with positive outcomes, in order to improve clinical management. The proposed study will investigate the relationships between dietary intake, changes in body composition, resting energy metabolism, and nutritional status after gastric bypass surgery. One of the primary hpotheses is that greater protein consumption in the first five to six weeks after surgery will be associated with more positive outcomes at 6 months and 1 year after surgery.